Sources of variation in response intervals after prostaglandin treatment in mares with functional corpora lutea

Sixty-two non-cycling mares were classified according to the size of largest follicles at the time of treatment with Prostalene, an analogue of prostaglandin (PG) F-2 alpha. Although oestrus occurred in only 77.4% of mares, 98.4% ovulated at an average of 6.8 days after treatment. Greatest variance of interval to ovulation was observed in mares having follicles greater than or equal to 40 mm at the time of treatment. This was due to regression of large follicles about one-third of the time and later ovulation of a succeeding follicle. This resulted also in greatest uncertainty of prediction of ovulation time based on ovarian palpation. Ninety foaling mares were given Prostalene at various days following the first ovulation post partum. Ovulation less than 6 days after treatment was strongly associated with the presence of a large follicle on the day of treatment. Otherwise most (72%) ovulations occurred 6--10 days after treatment. The distribution of interovulatory periods resulting from Prostalene on Day 6 after ovulation differed from that of Day 8 treatment.